Multiple-input-multiple-output antenna device

ABSTRACT

A Multiple-Input-Multiple-Output (MIMO) antenna device is adapted for connecting electrically to a radio frequency (RF) circuit for transmitting and receiving RF signals. The MIMO antenna device includes a circuit board, a plurality of antenna units, and a plurality of multiplexer units. The antenna units are disposed on the circuit board proximate to a peripheral edge thereof, are arranged in a loop formation, and are divided into a plurality of groups of the antenna units. Each of the multiplexer units is connected electrically to a respective one of the groups of the antenna units for selecting one of the corresponding antenna units and for connecting electrically the selected one of the corresponding antenna units to the RF circuit, thereby achieving the MIMO technique with the independently and simultaneously operating antenna units.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Application No.200910261357.4, filed on Dec. 22, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna device, more particularly toMultiple-Input-Multiple-Output (MIMO) antenna device.

2. Description of the Related Art

U.S. Patent Application Publication Nos. 20060109067 and 20060192720disclose two conventional smart array antenna systems, each of whichincludes a plurality of printed dipole antenna units that are large indimensions and that are coupled to a single Radio Frequency (RF) portvia a network comprising transmission cables, phase shifters, and powerdividers. Such arrangement of the smart array antenna system, thenetwork, and the RF port causes the printed dipole antenna units of thesmart array antenna system to be unable to operate independently. Theconventional smart array antenna systems are therefore unable to achievemaximum data rates between 450 Mbps and 600 Mbps (802.11n standard), andonly conform to the specifications of the 802.11a/b/g standards.Consequently, wireless access points or wireless routers that employ theconventional smart array antenna systems cannot utilize theMultiple-Input-Multiple-Output (MIMO) technique for increasedthroughput.

Hence, there is a need in the art to provide a MIMO antenna system thatcan be installed inside wireless access points or wireless routers, andthat includes antenna units adapted to operate independently so as toincrease transmission throughput by utilizing the MIMO technique suchthat the MIMO antenna system conforms with the 802.11a/b/g/n standards.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide aMultiple-Input-Multiple-Output (MIMO) antenna device with highdirectivity, high throughput, and multi-operation bands.

According to one aspect, a MIMO antenna device of the present inventionis adapted for connecting electrically to a radio frequency (RF) circuitso as to transmit and receive RF signals. The MIMO antenna deviceincludes a circuit board, a number (N) of antenna units, a(1×M)-multiplexer unit, and a (1×(N−M))-multiplexer unit. (N) is aninteger not smaller than 5, and (M) is an integer that is not smallerthan 2 and that is smaller than (N−1). The circuit board has aperipheral edge. The (N) antenna units are disposed on the circuit boardproximate to the peripheral edge of the circuit board, and are arrangedin a loop formation. The (1×M)-multiplexer unit is disposed on thecircuit board, is connected electrically to (M) corresponding ones ofthe (N) antenna units, and is operable to select one of the (M)corresponding ones of the antenna units for connecting electrically theselected one of the antenna units to the RF circuit. The(1×(N−M))-multiplexer unit is disposed on the circuit board, isconnected electrically to remaining (N−M) corresponding ones of the (N)antenna units, and is operable to select one of the (N−M) correspondingones of the antenna units for connecting electrically the selected oneof the antenna units to the RF circuit. Thus, by causing two of the (N)antenna units to operate independently, the MIMO antenna device is ableto utilize the MIMO technique so as to achieve high data rate andantenna radiation beamforming.

Preferably, the circuit board has a ground plane (e.g., a groundingunit). The (N) antenna units are disposed to surround the ground plane,are one-wavelength loop antennas having a balanced structure, and areoperable to suppress excited surface currents of the ground plane suchthat the ground plane acts as a reflector for improving radiationdirectivity and gain of radiation patterns of each of the antenna units.

Preferably, the MIMO antenna device further includes a controllerconnected electrically to the (1×M)-multiplexer unit and the(1×(N−M))-multiplexer unit for controlling operations thereof. The(1×M)-multiplexer unit is controlled by the controller to select one ofthe (M) corresponding ones of the antenna units for connectingelectrically the selected one of the antenna units to the RF circuit.The (1×(N−M))-multiplexer unit is controlled by the controller to selectone of the (N−M) corresponding ones of the antenna units for connectingelectrically the selected one of the antenna units to the RF circuit.

Preferably, to systematically control operations of the (N) antennaunits, the MIMO antenna device further includes a signal-sourcedetermining unit that is connected electrically to the controller andthat is adapted to be coupled to the RF circuit. The signal-sourcedetermining unit is operable to determine a source of a RF signal basedon intensity of the RF signal received by each of the (N) antenna unitsand to generate a control signal corresponding to the source of the RFsignal for controlling the controller. The controller is responsive tothe control signal so as to control: the (1×M)-multiplexer unit toselect one of the (M) corresponding ones of the (N) antenna units ableto receive a maximum intensity of the RF signal relative to other onesof the antenna units of the (M) corresponding ones of the (N) antennaunits for connecting electrically the selected one of the antenna unitsin the (M) corresponding ones of the (N) antenna units to the RFcircuit; and the (1×(N−M))-multiplexer unit to select one of the (N−M)corresponding ones of the (N) antenna units able to receive a maximumintensity of the RF signal relative to other ones of the antenna unitsin the (N−M) corresponding ones of the (N) antenna units for connectingelectrically the selected one of the antenna units in the (N−M)corresponding ones of the (N) antenna units to the RF circuit.

According to another aspect, a Multiple-Input-Multiple-Output (MIMO)antenna device of the present invention is adapted for connectingelectrically to a radio frequency (RF) circuit so as to transmit andreceive RF signals. The MIMO antenna device includes a circuit board, anumber (N) of antenna units, and three (1×(N/3))-multiplexer units. Thecircuit board has a peripheral edge. The (N) antenna units are disposedon the circuit board proximate to the peripheral edge of the circuitboard and are arranged in a loop formation. (N) is an integer notsmaller than 6 and is a multiple of 3. The three (1×(N/3))-multiplexerunits are disposed on the circuit board. Each of the(1×(N/3))-multiplexer units has (N/3) input terminals that are connectedelectrically and respectively to (N/3) corresponding ones of the (N)antenna units and are operable to select one of the (N/3) correspondingones of the (N) antenna units for connecting electrically the selectedone of the (N/3) corresponding ones of the (N) antenna units to the RFcircuit. Therefore, the MIMO antenna device is able to utilize the MIMOtechnique with a plurality of independently and simultaneously operatingantenna units.

Preferably, the MIMO antenna device of the present invention furtherincludes a controller connected electrically to the(1×(N/3))-multiplexer units. Each of the (1×(N/3))-multiplexer units iscontrolled by the controller to select one of the (N/3) correspondingones of the antenna units connected thereto for connecting electricallythe selected one of the antenna units to the RF circuit.

Preferably, the MIMO antenna device further includes a signal-sourcedetermining unit that is connected electrically to the controller andthat is adapted to be coupled to the RF circuit. The signal-sourcedetermining unit is operable to determine a source of a RF signal basedon intensity of the RF signal received by each of the (N) antenna units,and to generate a control signal corresponding to the source of the RFsignal for controlling the controller. The controller is responsive tothe control signal so as to control each of the (1×(N/3))-multiplexerunits to select one of the (N/3) corresponding ones of the (N) antennaunits able to receive a maximum intensity of the RF signal relative toother ones of the antenna units in the (N/3) corresponding ones of the(N) antenna units for connecting electrically the selected one of theantenna units to the RF circuit.

According to yet another aspect, Multiple-Input-Multiple-Output (MIMO)antenna device of the present invention is adapted for connectingelectrically to a radio frequency (RF) circuit so as to transmit andreceive RF signals. The MIMO antenna device includes a circuit board, aplurality of antenna units, and a plurality of multiplexer units. Thecircuit board has a peripheral edge. The antenna units are disposed onthe circuit board proximate to the peripheral edge of the circuit board,and are arranged in a loop formation. The antenna units are divided intoa plurality of groups of the antenna units. Each of the multiplexerunits is connected electrically to a respective one of the groups of theantenna units and is operable to select one of the antenna units in therespective one of the groups of the antenna units for connectingelectrically the selected one of the antenna units in the respective oneof the groups of the antenna units to the RF circuit. Therefore, theMIMO antenna device is able to utilize the MIMO technique using aplurality of independently and simultaneously operating antenna units.

Preferably, the MIMO antenna device further includes a controllerconnected electrically to the multiplexer units. Each of the multiplexerunits is controlled by the controller to select one of the antenna unitsin the respective one of the groups of the antenna units for connectingelectrically the selected one of the antenna units in the respective oneof the groups of the antenna units to the RF circuit.

Preferably, the MIMO antenna device further includes a signal-sourcedetermining unit that is connected electrically to the controller andthat is adapted to be coupled to the RE circuit. The signal-sourcedetermining unit is operable to determine a source of a RF signal basedon intensity of the RF signal received by each of the antenna units, andto generate a control signal corresponding to the source of the RFsignal for controlling the controller. The controller is responsive tothe control signal so as to control each of the multiplexer units toselect one of the antenna units that is in the respective one of thegroups of the antenna units and that is able to receive a maximumintensity of the RF signal relative to other ones of the antenna unitsin the respective one of the groups of the antenna units for connectingelectrically the selected one of the antenna units in the respective oneof the groups of the antenna units to the RF circuit.

Through disposing the antenna units on the circuit board, arranging theantenna units in a loop formation, and connecting electrically each ofthe antenna units to a respective one of the multiplexer units, each ofthe multiplexer units is operable to select one of the correspondingones of the antenna units for connecting electrically the selected oneof the corresponding ones of the antenna units to the RF circuit. Hence,the RF circuit is able to transmit and receive RF signals via at leasttwo independently operating antenna units so as to utilize the MIMOtechnique such that wireless broadband access points or wireless routersemploying the MIMO antenna device of the present invention is capable ofachieving high directivity, high throughput, and antenna radiationbeamforming.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent in the following detailed description of the preferredembodiments with reference to the accompanying drawings, of which:

FIG. 1 is a functional block diagram of the first preferred embodimentof a Multiple-Input-Multiple-Output (MIMO) antenna device according tothe present invention;

FIG. 2 is a schematic view of a plurality of antenna units and a circuitboard of the MIMO antenna device of the first preferred embodiment;

FIG. 3 is a schematic diagram illustrating that each set of antennaunits of the MIMO antenna device of the first preferred embodimentcovers an included angle of 60°;

FIG. 4 is a schematic view of one of the antenna units of the MIMOantenna device of the first preferred embodiment;

FIG. 5 is a schematic diagram illustrating that each set of antennaunits of the MIMO antenna device of the first preferred embodimentcovers a an included angle of 40°;

FIG. 6 is a schematic diagram illustrating that each of(1×6)-multiplexer units of the MIMO antenna device of the firstpreferred embodiment can include one (1×2)-multiplexer and two(1×3)-multiplexers;

FIG. 7 is a schematic diagram illustrating that each of(1×6)-multiplexer units of the MIMO antenna device of the firstpreferred embodiment can include one (1×3)-multiplexer and three(1×2)-multiplexers;

FIG. 8 is a functional block diagram of the second preferred embodimentof a MIMO antenna device according to the present invention;

FIG. 9 is a schematic diagram illustrating that each set of antennaunits of the MIMO antenna device of the second preferred embodimentcovers an included angle of 60°;

FIG. 10 is a functional block diagram of the third preferred embodimentof a MIMO antenna device according to the present invention;

FIG. 11 is a schematic diagram illustrating that each set of antennaunits of the MIMO antenna device of the third preferred embodimentcovers an included angle of 120°;

FIG. 12 is a functional block diagram of the fourth preferred embodimentof a MIMO antenna device according to the present invention; and

FIG. 13 is a schematic diagram illustrating arrangement of antenna unitsof the MIMO antenna device of the fourth preferred embodiment on acircuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the present invention is described in greater detail, it shouldbe noted that like elements are denoted by the same reference numeralsthroughout the disclosure.

Referring to FIGS. 1 and 2, the first preferred embodiment of aMultiple-Input-Multiple-Output (MIMO) antenna device 1 according to thepresent invention is adapted to be connected electrically to a RadioFrequency (RF) circuit 2 for transmitting and receiving RF signals. TheMIMO antenna device 1 includes a circuit board 10, a number (N) ofantenna units ((N) is an integer not smaller than 6 and is a multiple of3), and a plurality of (1×(N/3))-multiplexer units. In the presentembodiment, (N) is equal to 18, and the antenna units are dividedequally into first, second, and third groups of antenna units (A1-F1),(A2-F2), (A3-F3). Each of the groups of antenna units is connectedelectrically to a corresponding one of (1×6)-multiplexer units 11, 12,13.

The circuit board 10 is a multi-layer board, and at least one of thelayers of the circuit board 10 is provided with a ground plane 100(e.g., a grounding unit). In the present embodiment, the ground plane100 is disposed on a surface of the circuit board 10.

The antenna units of the first, second, and third groups of antennaunits (A1-F1), (A2-F2), (A3-F3) are vertically disposed on the circuitboard 10 proximate to a peripheral edge of the same, are equiangularlyspaced apart from adjacent ones thereof, are arranged in a loopformation, and are disposed to surround the ground plane 100. Theantennas units of the first, second, and third groups of antenna units(A1-F1), (A2-F2), (A3-F3) can be divided into six sets of antenna units(A1-A3), (B1-B3), (C1-C3), (D1-D3), (E1-E3), and (F1-F3), and aredisposed on the circuit board 10 in the order given. Referring to FIG.3, an angle between adjacent ones of the antenna units is 20° such thateach of the antenna units is arranged to transmit and receive RF signalstoward and from a unique direction with respect to a horizontal plane.Thus, each of the sets of antenna units (A1-A3), (B1-B3), (C1-C3),(D1-D3), (E1-E3), (F1-F3) covers an included angle (or beam angle) of60°.

When the antenna device 1 of the first preferred embodiment isconfigured to operate in a 802.11n network, each of the antenna units ofa selected one of the sets of antenna units (A1-A3), (B1-B3), (C1-C3),(D1-D3), (E1-E3), (F1-F3) will be able to achieve a maximum transmissionrate of 150 Mbps. That is to say, the MIMO antenna device 1 of the firstpreferred embodiment will be able to achieve a maximum transmission rateof 450 Mbps when the antenna units of the selected one of the sets ofantenna units (A1-A3), (B1-B3), (C1-C3), (D1-D3), (E1-E3), (F1-F3) areconfigured to transmit or receive RF signals simultaneously. Similarly,a maximum data rate of 600 Mbps can be achieved if each set of antennaunits includes four antenna units.

Referring to FIG. 4, the antenna units of the MIMO antenna device 1 ofthe present embodiment are one-wavelength loop antennas. Each of theantenna units includes a dielectric substrate 30 that can be made of aceramic material or that can be a substrate made of fiberglassreinforced epoxy resin or the like, and first and second loop antennas31, 32. The first and second loop antennas 31, 32 are disposed on thesubstrate 30, are bent along surfaces of the substrate 30 to conformthereto, are different from each other in terms of dimensions (in thisembodiment, dimensions of the first loop antennas 31 are larger thanthose of the second loop antennas 32), and are operable in first (e.g.,a lower frequency band centered on 2.4 GHz) and second (e.g., a higherfrequency band centered on 5 GHz) frequency bands, respectively.

The first loop antenna 31 has a first feed portion 33 and a first groundportion 34 that are substantially parallel to each other. The first feedportion 33 is adapted to be coupled to the RF circuit 2 so as to feed RFsignals, and the first ground portion 34 is connected electrically tothe ground plane 100. The second loop antenna 32 has a second feedportion 36 that is adapted to be coupled to the RF circuit 2 so as tofeed RF signals, and a second ground portion 35 that is connectedelectrically to the ground plane 100. The second feed portion 36 and thesecond ground portion 35 are substantially parallel to each other, andthe former and the latter are connected electrically to the first feedportion 33 and the first ground portion 34 of the first loop antenna 31,respectively.

For each of the antenna units, the first and second feed portions 33, 36are relatively proximate to one of two adjacent ones of the antennaunits. The first and second feed portions 33, 36 of each of the antennaunits are adjacent to the first and second ground portions 34, 35 of thesame antenna unit, and are adjacent to the first and second groundportions 34, 35 of said one of two adjacent ones of the antenna unitssuch that each first feed portion 33 is interposed between two firstground portions 34, and that each second feed portion 36 is interposedbetween two second ground portions 35. Such arrangement reducesinterference between the first feed portions 33 and between the secondfeed portions 36 of the antenna units, and ensures that mutual couplingbetween adjacent ones of the antenna units is below −20 dB.

Moreover, by virtue of bending of the first and second loop antennas 31,32 and use of the dielectric substrate 30, the antenna units are able tohave relatively small dimensions such that the MIMO antenna device 1 canbe disposed inside wireless access points or wireless routers, therebyavoiding compromised aesthetics and reducing vulnerability to externalforce.

Furthermore, each of the one-wavelength loop antennas has a balancedstructure for suppressing extensively excited surface currents of theground plane 100 such that the ground plane 100 can act as a reflectorfor improving radiation directivity and gain of radiation patterns ofthe antenna units.

The (1×6)-multiplexer units 11, 12, 13 can be disposed on anothersurface of the circuit board 10 opposite to the ground plane 100.However, arrangement of the (1×6)-multiplexer units 11, 12, 13 is notlimited to such. The (1×6)-multiplexer units 11, 12, 13 are adapted tobe connected electrically to the RF circuit 2. Referring to FIG. 1, eachof the (1×6)-multiplexer units 11, 12, 13 has six input terminals, eachof the six input terminals is connected electrically to at least one ofthe first and, second feed portions 33, 36 of a corresponding one of theantenna units, and each of the (1×6)-multiplexer units 11, 12, 13 isoperable to select one of the six corresponding ones of the antennaunits connected thereto for connecting electrically the selected one ofthe six corresponding ones of the antenna units to the RF circuit 2.

By controlling each of the (1×6)-multiplexer units 11, 12, 13 to selectone of the six corresponding ones of the antenna units and to connectelectrically the selected one of the six corresponding ones of theantenna units to the RF circuit 2, and by configuring the RF circuit 2to transmit and receive RF signals via each of the selected ones of theantenna units connected thereto, the MIMO antenna device 1 is able toutilize the MIMO technique and is operable to achieve antenna radiationbeamforming. The antenna units of the present invention are used in amore efficient manner that a combined radiation pattern is directedtoward an intended receiver, thereby ensuring quality of communicationand increasing range of coverage.

The antenna device 1 further includes a controller 14 and asignal-source determining unit 15 for controlling the (1×6)-multiplexerunits 11, 12, 13 so as to control selection of the antenna units.

The signal-source determining unit 15 is adapted to be coupled to the RFcircuit 2, and is connected electrically to the controller 14. Thesignal-source determining unit 15 is operable to determine a location ofa source of a received RF signal based on intensity of the RF signalreceived by each of the antenna units (e.g., cause the RF circuit 2 toreceive the RF signal via each of the antenna units), and is operable togenerate a control signal corresponding to the determined location ofthe source of the received RF signal for controlling the controller 14.The controller 14 is responsive to the control signal so as to controleach of the (1×6)-multiplexer units 11, 12, 13 to select one of thecorresponding ones of the antenna units able to receive a maximumintensity of the RF signal relative to other ones of the correspondingones of the antenna units for connecting the selected one of the antennaunits to the RF circuit 2. For example, the signal-source determiningunit 15 will cause the controller 14 to control the (1×6)-multiplexerunits 11, 12, 13 to select antenna units A1, A2, A3, and to connectelectrically the selected antenna units A1, A2, A3 to the RF circuit 2when each of antenna units A1, A2, A3 is able to receive a relativelymaximum intensity of a RF signal, and will further cause the controller14 to control the (1×6)-multiplexer units 11, 12, 13 to select antennaunits C1, C2, C3 when the source of the RF signal moves to a locationwhere each of antenna units C1, C2, C3 is able to receive a relativelymaximum intensity of the RF signal. Therefore, the MIMO antenna device 1is operable to radiate a directional radiation pattern and to direct thebeam-shaped radiation pattern toward an intended receiver for farthercommunications range. Therefore, the MIMO antenna device 1 can be usedin a Wireless Local Area Network (802.11a/b/g/n) for achieving highdirectivity and high data rate.

Besides, when the directivity is not taken into consideration, the(1×6)-multiplexer units 11, 12, 13 do not necessarily have to selectantenna units of a same set of antenna units (e.g., A1, A2, A3) or threeconsecutive antenna units (e.g., C1, C2, B3). In other words, the(1×6)-multiplexer units 11, 12, 13 can select three antenna units thatare not adjacent to each other so as to transmit and receive RF signalsin different directions or regions.

In addition, to further improve directivity of the antenna units, thecontroller 14 is operable to control only two of the (1×6)-multiplexerunits 11, 12, 13 to operate such that each of said two of the(1×6)-multiplexer units 11, 12, 13 is controlled to select one of thesix corresponding ones of the antenna units and to connect electricallythe selected corresponding one of the antenna units to the RF circuit 2.In other words, only two adjacent ones of the antenna units are used fortransmitting and receiving RF signals simultaneously. Hence, as shown inFIG. 5, the antenna units of the first, second, third groups of antennaunits (A1-F1), (A2-F2), (A3-F3) can be divided into nine sets of antennaunits (A1, A2), (A3, B1), (B2, B3), (C1, C2), (C3, D1), (D2, D3), (E1,E2), (E3, F1), and (F2, F3), each set of antenna units being operable toutilize the MIMO technique and to cover an included angle of 40° forimproving directivity of the antenna units.

Referring to FIGS. 6 and 7, each of the (1×6)-multiplexer units 11, 12,13 can be implemented using two (1×3)-multiplexers and one(1×2)-multiplexer, or using three (1×2)-multiplexers and one(1×3)-multiplexer.

Referring to FIG. 8, the second preferred embodiment of a MIMO antennadevice 1 according to the present invention includes a number (N) ofantenna units (N is not smaller than 5) divided into first and secondgroups of antenna units, a (1×M)-multiplexer unit, and a(1×(N−M))-multiplexer unit (M is not smaller than 2 and is smaller than(N−1)). In the present embodiment, the first and second groups ofantenna units include (M) and (N−M) antenna units, respectively. (N) and(M) are equal to 12 and 6, respectively. Thus, the MIMO antenna device 1includes first and second groups of antenna units (A1-F1), (A2-F2), andfirst and second (1×6)-multiplexer units 71, 72. Referring to FIG. 9,the first and second groups of antenna units (A1-F1), (A2-F2) aredisposed on a circuit board 70 in a loop formation and in the ordergiven, and the first and second groups of antenna units (A1-F1), (A2-F2)are connected electrically and respectively to the first and second(1×6)-multiplexer units 71, 72.

The first and second (1×6)-multiplexers units 71, 72 are controlled by acontroller 14 such that each of the first and second (1×6)-multiplexerunits 71, 72 is controlled to select one of the six corresponding onesof the antennas units for connecting electrically the selected one ofthe six corresponding ones of the antenna units to the RF circuit 2.Thus, as shown in FIG. 9, the antenna units of the first and secondgroups of antenna units (A1-F1) (A2-F2) can be divided into six sets ofantenna units (A1, A2), (B1, B2), (C1, C2), (D1, D2), (E1, E2), and (F1,F2). Each of the sets is operable to utilize the MIMO technique andcovers an included angle of 60°. The other components and operationsthereof in the present embodiment are identical to those in the firstpreferred embodiment, and hence will not be described hereinafter forthe sake of brevity.

Referring to FIG. 10, the third preferred embodiment of a MIMO antennadevice 1 according to the present invention includes first and second(1×3)-multiplexer units 81, 82, and first and second groups of antennaunits (A1, B1, C1), (A2, B2, C2). Positions of the first and secondgroups of antenna units (A1, B1, C1), (A2, B2, C2) with respect to acircuit board 80 are as shown in FIG. 11. The first and second(1×3)-multiplexer units 81, 82 are connected electrically andrespectively to the first and second groups of antenna units (A1, B1,C1), (A2, B2, C2). Each of the first and second groups of antenna units(A1, A2, B1), (B2, C1, C2) covers an included angle of 180°. Acontroller 83 is connected to the first and second (1×3)-multiplexerunits 81, 82 and is responsive to a control signal generated by asignal-source determining unit 84 for controlling each of the(1×3)-multiplexer units 81, 82 to select one of the three correspondingones of the antenna units and to connect electrically the selected oneof the three corresponding ones of the antenna units to the RF circuit 2so as to enable transmission and receipt of RF signals via one of thefirst and second groups of antenna units (A1, A2, B1) and (B2, C1, C2).

Of course, the first and second (1×3)-multiplexer units 81, 82 of thethird preferred embodiment can be replaced by first, second, and third(1×2)-multiplexer units. The antenna units of the first and secondgroups of antenna units (A1, B1, C1), (A2, B2, C2) can be divided intofirst, second, and third sets of antenna units (A1, A2), (B1, B2), (C1,C2) - - - or (A2, B1), (B2, C1), (C2, A1). The first, second, and third(1×2)-multiplexer units are connected electrically and respectively tothe first, second, and third sets of antenna units (A1, B2), (A2, C1),(B1, C2). Each of first, second, and third sets of antenna units (A1,B2), (A2, C1), (B1, C2) is operable to utilize the MIMO technique andcovers an included angle of 120°. Hence, the MIMO antenna device 1 ofthe third preferred embodiment is operable to utilize the MIMO techniquewith a minimum number of antenna units.

Referring to FIG. 12, the fourth preferred embodiment of a MIMO antennadevice 1 according to the present invention is based on the secondpreferred embodiment. However, (N) in the present embodiment is equal to5, i.e., the minimum number of antenna units required for achievingfarther communications range with omnidirectional coverage andimplementing the MIMO technique for higher data rate.

In the present embodiment, a (1×2)-multiplexer unit 51 has inputterminals connected electrically and respectively to a first group ofantenna units (A1, B1), and a (1×3)-multiplexer unit 52 has inputterminals connected electrically and respectively to a second group ofantenna units (A2, B2, C1). The positions of the first and second groupsof antenna units (A1, B1), (A2, B2, C1) with respect to a circuit board50 are as shown in FIG. 13. The MIMO antenna device 1 of the presentembodiment includes a controller 53 for controlling each of the(1×2)-multiplexer unit 51 and the (1×3)-multiplexer unit 52 to selectone of the corresponding antenna units of a respective one of the firstand second groups of antenna units (A1, B1), (A2, B2, C1) and to connectelectrically the selected one of the corresponding antenna units to theRF circuit 2. The MIMO antenna device 1 of the fourth preferredembodiment is thus able to achieve beamforming and to utilize the MIMOtechnique with a minimum number of antenna units.

It is to be noted that single-output multiplexers are used in theabove-mentioned preferred embodiments. However, in practice, ifproduction cost is not a major consideration,multiple-input-multiple-output multiplexers can also be employed. Forexample, if the MIMO antenna device 1 includes 16 antenna units, andeach transmission and reception of RF signals requires four of theantenna units, the MIMO antenna device 1 can employ two(2×8)-multiplexer units. Each of the two (2×8)-multiplexer units isconnected electrically to eight corresponding ones of the antenna unitsfor selecting two of the eight corresponding ones of the antenna unitsconnected thereto and for connecting electrically the selected two ofthe eight corresponding ones of the antenna units to the RF circuit 2.The MIMO antenna device 1 can thus utilize the MIMO technique, and issuitable to be used in a 802.11 a/b/g/n network for increasingthroughput.

In summary, through disposing the antenna units on the circuit board,arranging the antenna units in a loop formation, and connectingelectrically each of the antenna units to a respective one of themultiplexer units, each of the multiplexer units is operable to selectone of the corresponding ones of the antenna units for connectingelectrically the selected one of the corresponding ones of the antennaunits to the RF circuit. Thus, the RF circuit is able to transmit andreceive RF signals simultaneously via at least two independentlyoperating antenna units so as to utilize the MIMO technique. Therefore,wireless broadband access points or wireless routers that employ theMIMO antenna device of the present invention are able to achieve highdirectivity, high throughput, and farther communications range withomnidirectional coverage.

While the present invention has been described in connection with whatare considered the most practical and preferred embodiments, it isunderstood that this invention is not limited to the disclosedembodiments but is intended to cover various arrangements includedwithin the spirit and scope of the broadest interpretation so as toencompass all such modifications and equivalent arrangements.

What is claimed is:
 1. A multiple-input multiple-output (MIMO) antenna device adapted for connecting electrically to a radio frequency (RF) circuit so as to transmit and receive RF signals, said MIMO antenna device comprising: a circuit board having a peripheral edge; a number (N) of antenna units disposed on said circuit board proximate to said peripheral edge of said circuit board and arranged in a loop formation, wherein the number (N) is a positive integer; a (1×M)-multiplexer unit disposed on said circuit board, connected electrically to a number (M) of corresponding ones of said (N) antenna units, and operable to select one of said (M) corresponding ones of said antenna units for connecting electrically the selected one of said antenna units to the RF circuit, wherein the number (M) is a positive integer; and a (1×(N−M))-multiplexer unit disposed on said circuit board, connected electrically to remaining (N−M) corresponding ones of said (N) antenna units, and operable to select one of said (N−M) corresponding ones of said antenna units for connecting electrically the selected one of said antenna units to the RF circuit; wherein said circuit board has a ground plane, and said (N) antenna units are disposed to surround said ground plane.
 2. The MIMO antenna device as claimed in claim 1, further comprising a controller connected electrically to said (1×M)-multiplexer unit and said (1×(N−M))-multiplexer unit, said (1×M)-multiplexer unit being controlled by said controller to select one of said (M) corresponding ones of said antenna units for connecting electrically the selected one of said antenna units to the RF circuit, said (1×(N−M))-multiplexer unit being controlled by said controller to select one of said (N−M) corresponding ones of said antenna units for connecting electrically the selected one of said antenna units to the RF circuit.
 3. The MIMO antenna device as claimed in claim 2, further comprising a signal-source determining unit that is connected electrically to said controller and that is adapted to be coupled to the RF circuit, said signal-source determining unit being operable to determine a source of a RF signal based on intensity of the RF signal received by each of said (N) antenna units and to generate a control signal corresponding to the source of the RF signal for controlling said controller, said controller being responsive to the control signal so as to control said (1×M)-multiplexer unit to select one of said (M) corresponding ones of said antenna units able to receive a maximum intensity of the RF signal relative to other ones of said antenna units in said (M) corresponding ones of said antenna units for connecting electrically the selected one of said antenna units in said (M) corresponding ones of said antenna units to the RF circuit, and said (1×(N−M))-multiplexer unit to select one of said (N−M) corresponding ones of said antenna units able to receive a maximum intensity of the RF signal relative to other ones of said antenna units in said (N−M) corresponding ones of said antenna units for connecting electrically the selected one of said antenna units in said (N−M) corresponding ones of said antenna units to the RF circuit.
 4. The MIMO antenna device as claimed in claim 1, wherein each of said (N) antenna units is a one-wavelength loop antenna and includes a dielectric substrate that has a surface, and a first loop antenna and a second loop antenna that are disposed on said surface of said dielectric substrate, said first and second loop antennas being operable in first and second frequency bands, respectively.
 5. The MIMO antenna device as claimed in claim 4, wherein said first loop antenna has a first feed portion and a first ground portion, said first feed portion being substantially parallel to said first ground portion, said first feed portion being adapted to be coupled to the RF circuit so as to permit feeding of the RF signal therebetween, said first ground portion being connected electrically to said ground plane of said circuit board.
 6. The MIMO antenna device as claimed in claim 4, wherein said second loop antenna has a second feed portion adapted to be coupled to the RF circuit so as to permit feeding of the RF signal therebetween the RF signal, and a second ground portion connected electrically to said ground plane of said circuit board, said second feed portion being substantially parallel to said second ground portion.
 7. The MIMO antenna device as claimed in claim 1, wherein (N) is an integer not smaller than 5, and (M) is an integer that is not smaller than 2 and that is smaller than (N−1).
 8. The MIMO antenna device as claimed in claim 1, wherein (N) is equal to 5, and (M) ranges from 1 to
 4. 9. The MIMO antenna device as claimed in claim 1, wherein (N) is equal to 6 and (M) is equal to
 3. 10. The MIMO antenna device as claimed in claim 1, wherein (N) is equal to 12 and (M) is equal to 6, said (1×M)-multiplexer unit being a (1×6)-multiplexer unit having six corresponding ones of said antenna units connected electrically thereto, said (1×(N−M))-multiplexer unit being a (1×6)-multiplexer unit having another six corresponding ones of said antenna units connected electrically thereto.
 11. The MIMO antenna device as claimed in claim 10, wherein each of said (1×6)-multiplexer units includes a (1×2)-multiplexer and two (1×3)-multiplexers connected to said (1×2)-multiplexer, each of said (1×3)-multiplexers being connected electrically to three corresponding ones of said antenna units and being operable to select one of said three corresponding ones of said antenna units for connecting electrically the selected one of said three corresponding ones of said antenna units to said (1×2)-multiplexer, said (1×2)-multiplexer being operable to select one of said antenna units connected electrically thereto via said (1×3)-multiplexers for connecting electrically said antenna unit selected thereby to the RF circuit.
 12. The MIMO antenna device as claimed in claim 10, wherein each of said (1×6)-multiplexer units includes a (1×3)-multiplexer and three (1×2)-multiplexers connected to said (1×3)-multiplexer, each of said (1×2)-multiplexers being connected electrically to two corresponding ones of said antenna units and being operable to select one of said two corresponding ones of said antenna units for connecting electrically the selected one of said two corresponding ones of said antenna units to said (1×3)-multiplexer, said (1×3)-multiplexer being operable to select one of said antenna units connected electrically thereto via said (1×2)-multiplexers for connecting electrically said antenna unit selected thereby to the RF circuit.
 13. A multiple-input multiple-output (MIMO) antenna device adapted for connecting electrically to a radio frequency (RF) circuit so as to transmit and receive RF signals, said MIMO antenna device comprising: a circuit board having a peripheral edge; a plurality of antenna units disposed on said circuit board proximate to said peripheral edge of said circuit board and arranged in a loop formation, said antenna units being divided into a plurality of groups of said antenna units; and a plurality of multiplexer units, each of which is connected electrically to a respective one of said groups of said antenna units and is operable to select one of said antenna units in the respective one of said groups of said antenna units for connecting electrically the selected one of said antenna units in the respective one of said groups of said antenna units to the RF circuit; wherein said circuit board has a ground plane, and said antenna units are disposed to surround said ground plane.
 14. The MIMO antenna device as claimed in claim 13, further comprising a controller connected electrically to said multiplexer units, each of said multiplexer units being controlled by said controller to select one of said antenna units in the respective one of said groups of said antenna units for connecting electrically the selected one of said antenna units in the respective one of said groups of said antenna units to the RF circuit.
 15. The MIMO antenna device as claimed in claim 14, further comprising a signal-source determining unit that is connected electrically to said controller and that is adapted to be coupled to the RF circuit, said signal-source determining unit being operable to determine a source of a RF signal based on intensity of the RF signal received by each of said antenna units, and to generate a control signal corresponding to the source of the RF signal for controlling said controller, said controller being responsive to the control signal to control each of said multiplexer units to select one of said antenna units that is in the respective one of said groups of said antenna units and that is able to receive a maximum intensity of the RF signal relative to other ones of said antenna units in the respective one of said groups of said antenna units for connecting electrically the selected one of said antenna units in the respective one of said groups of said antenna units to the RF circuit.
 16. The MIMO antenna device as claimed in claim 13, wherein each of said antenna units is a one-wavelength loop antenna and includes a dielectric substrate that has a surface, and a first loop antenna and a second loop antenna that are disposed on said surface of said dielectric substrate, said first and second loop antennas being operable in first and second frequency bands, respectively.
 17. The MIMO antenna device as claimed in claim 16, wherein said first loop antenna has a first feed portion and a first ground portion, said first feed portion being substantially parallel to said first ground portion, said first feed portion being adapted to be coupled to the RF circuit so as to permit feeding of the RF signal therebetween, said first ground portion being connected electrically to said ground plane of said circuit board.
 18. The MIMO antenna device as claimed in claim 16, wherein said second loop antenna has a second feed portion adapted to be coupled to the RF circuit so as to permit feeding of the RF signal therebetween, and a second ground portion connected electrically to said ground plane of said circuit board, said second feed portion being substantially parallel to said second ground portion. 